The present invention relates generally to forming of materials, and more particularly to a method and apparatus for the superplastic forming of materials, the method and apparatus including a preforming operation.
Superplastic alloys have long been known to exhibit large strains to failure and strong resistance to necking during tensile elongation. Superplastic forming (xe2x80x9cSPFxe2x80x9d) has been developed as an effective way to form such alloys and offers several advantages over conventional stamping techniques including increased formability, zero springback, and low tooling costs. The large degree of plastic strain that can be achieved with this process ( greater than 200%) makes it possible to form complex parts that cannot be shaped with conventional stamping techniques. These alloys can be formed with relatively low forces and they permit a high level of detail in the stamping design.
Typical superplastic forming takes place in a simple one-sided, single action tool. The blank is clamped in a heated die and then blow formed with gas pressure into a female die. The part detail is captured within a single die rather than a matched pair and therefore tooling is significantly less expensive than that of conventional stamping. Furthermore, the low forces needed to form the material at these elevated temperatures allows for the use of cast iron dies instead of the harder to work and more expensive tool steel.
While superplastic forming may be a viable manufacturing option for some parts, there are limitations in the economic feasibility of this technique. Superplastic response in metals is inherently coupled with the rate of deformation and there exists only a narrow range of strain rates, typically slow strain rates, in which these materials display superplastic response. This results in a relatively slow cycle time which often leaves superplastic forming as a cost prohibitive option for high volume parts.
Another problem related to SPF stems from the inability to draw material into the die cavity. Although the superplastic material utilized in SPF can undergo substantial deformation, its formability is limited to the amount of material in the die. After the die faces are clamped and sealed, additional superplastic material cannot be drawn into the die. This may result in tears or inconsistent wall thickness in the part being formed. To overcome this, U.S. Pat. No. 5,974,847 introduces preforming the material around a punch before gas pressure sealing the dies and completing the forming process by gas pressure injection. This approach reduces the amount of superplastic forming that takes place thereby reducing the cycle time and potentially allowing greater design freedom due to the additional material drawn into the die during the preforming step. While the method of this patent teaches preforming the material before the gas is injected, the method does not restrain the material entering the die during the preforming step. Without a restraining force on the material, such as blankholder force, the material will wrinkle around the punch in all but the simplest of formings. Wrinkling of the material during preforming will result in either the inability to complete the part during subsequent gas pressure forming or, at best, a low quality finished part.
Therefore, there exists a need for a method of forming superplastic materials which controls the amount of material to be drawn into the die cavity during a preforming process so as to avoid wrinkling of the material. The present invention provides such a need by controlling the material flow into the die during the preforming step, thereby eliminating wrinkles in the preformed part. An apparatus to adjust the amount of material flowing into the die is also necessary to ensure uniform preform wall thicknesses and high quality preformed parts. These preforms will lead to more consistent finished parts and will assist in increasing the speed of the forming process.
It is an advantage of the present invention to provide a method of superplastic forming which increases forming speed while reducing surface defects in the formed part.
It is another advantage of the present invention to provide a method of reducing tooling cost by using one-sided cast iron dies and providing a universal lower die system that can be used to form a variety of parts.
It is another advantage of the present invention to provide a method of restraining the sheet during the preforming step so as to produce a blankholder effect that prevents wrinkling of the sheet.
The present invention provides these advantages by providing a method of shaping a metal sheet into a formed product, comprising the steps of providing a first and second die member operative to move between a first open position and a second sealed position such that a die cavity is formed; providing a preforming punch disposed on one of the die members; providing a metal sheet of ductile material; and providing a blankholder engagable with a cushion system operative to move between a first material loading position and a second material loaded position. The method further comprises the steps of heating the die members and the preforming punch to a predetermined temperature, heating the metal sheet to a predetermined material forming temperature and moving the first and second die members to the open position and the blankholder to the material loading position. The method then continues with the steps of placing the metal sheet into the blankholder, moving a die member to engage the blankholder until the metal sheet contacts the preforming punch. The method further includes the steps of moving a die member, the blankholder, and the metal sheet until the die member sealed position is reached, controlling the amount of material flow into the die cavity as the metal sheet is over the preforming punch and applying gas pressure to the metal sheet after the sealed position is reached and until forming of the product is completed. Once completed, the die members are opened and the finished part removed.
The present invention further provides an apparatus for use with the method of the present invention, the apparatus comprising at least two die members operative to move between a first open position and second sealed position such that a die cavity is formed; a preforming punch disposed upon one of the die members, a cushioning system operative to control the amount of material flow into the die cavity as the metal sheet is formed over the preforming punch and a blankholder engagable with the cushion system. The apparatus further comprises a heating platen operative to raise the temperature of the die members to a predetermined level; a source of gas pressure and passages for directing the gas pressure into the die cavity; and wherein the cushioning system exerts a force on the metal sheet holding the metal sheet in place and permitting controlled material flow into the die until the second sealed position is reached. The present invention provides these advantages by utilizing a die cushioning system to control material flow into the die cavity during the preforming step reducing wrinkling in the preform and finished part.